Insect and Disease Management in Organic Crop Systems

Avoidance Techniques Managing the Growth Environment - Giving the Crop a Head Start Direct Treatment Conclusion References

Managing the ecosystem on an organic farm is very challenging.  It is made even more complex when factoring in insect and disease pests.

Since the use of synthetic pesticides are prohibited, the organic cropping system should be focused on the prevention of pest outbreaks rather than coping with them after they occur. No single method is likely to be adequate for all pests. Successful pest management depends on the incorporation of a number of control strategies. Some strategies will target insect and disease pests separately and others will target them together.

Pests in a crop do not automatically result in damage or yield loss. In some instances, low levels of insect feeding have been shown to increase crop yields. Once infestation levels reach a certain point, however, they can produce economic losses. Thresholds vary with the crop and the pest in question and must be closely monitored by the producer.

Planning for effective insect and disease management must involve the entire farm operation and use all information available. (See MAFRI Guide to Crop Protection for detailed information on thresholds and economic damage from various insects and diseases.)

Any strategy an organic producer uses should include methods for:

• insect and disease avoidance
• managing the growth environment
• direct treatment
   

Avoidance Techniques

To manage pests effectively, producers need to understand the biology and growth habits of both pest and crop. The type and concentration of pests are often responses to previous crop history, pest life cycles, soil conditions and local weather patterns.

Crop Rotations

Crop rotation is central to all sustainable farming systems. It is an extremely effective way to minimize most pest problems while maintaining and enhancing soil structure and fertility.

Diversity is the key to a successful crop rotation program.  It involves:

• rotating early-seeded, late-seeded and fall-seeded crops

• rotating between various crop types, such as annual, winter annual, perennial, grass and broadleaf crops; each of these plant groups has specific rooting habits, competitive abilities, nutrient and moisture requirements. (True diversity does not include different species within the same family - for example, wheat, oats and barley are all species of annual cereals.)

• incorporating green manure crops, such as sweet clover and peas, into the soil to suppress pests, disrupt their life cycles and to provide the additional benefits of fixing nitrogen and improving soil properties

• managing the frequency with which a crop is grown within a rotation

• maintaining the rotation's diversified habitat, which provides parasites and predators of pests with alternative sources of food, shelter and breeding sites

• planting similar crop species as far apart as possible. Insects such as wheat midge and Colorado potato beetle, for example, are drawn to particular host crops and may overwinter in or near the previous host crops. With large distances to move to get to the successive crop, the insects' arrival may be delayed. The number that find the crop may be reduced as well.

• Diverse rotations are particularly effective in regulating flea beetles, cabbage butterfly, wheat midge, wheat stem maggot and wheat stem sawfly.

Rotations are also effective in controlling soil- and stubble-borne diseases. The success of rotations in preventing disease depends on many factors, including the ability of a pathogen to survive without its host and the pathogen's host range. Those with a wide range of hosts will be controlled less successfully. For example, sclerotinia stem-rot is a common disease in conventionally grown canola on the Prairies, but it can also infect at least a half­dozen other field crops. Rotations will not have much effect on pathogens that live indefinitely in the soil, but will shorten the life span of pathogens that can survive only brief periods apart from their hosts. Other situations that limit the benefit of crop rotations include: the transmission of pathogens via seed, the presence of susceptible weeds and volunteer crops that harbour pathogens, and the invasion of pathogens by wind and other means.

Rotations should be used with other cultural practices to achieve the greatest benefit.
 

Ergot in rye seed Photo credit: Canadian Phytopathological Society

Ergot in rye head Photo credit: Canadian Phytopathological Society

Field Sanitation/Crop Residue Management

Reducing or removing crop residues and alternate host sites can be used to control some insects and many diseases. Incorporating the residue into the soil hastens the destruction of disease pathogens by beneficial fungi and bacteria. Burying diseased plant material in this manner also reduces the movement of spores by wind.

Insects most affected by tillage will be those that overwinter in crop residue (for example, European corn borer and wheat stem sawfly) and those that lay their eggs in the residue. Conversely, fields where residue has not been disturbed may have higher levels of some beneficial predaceous insects, which may reduce levels of insect pests such as root maggots in canola. Reduced or zero-tillage may also reduce the damage by certain pests, as the crop residue creates a micro-climate less preferred by some insects (for example, flea beetles).
 

Beneficial insect: ladybug larva Photo credit: Roy Ellis

Beneficial insect: ladybug adult Photo credit: Roy Ellis

It is important to maintain a balance between crop sanitation and soil conservation. Lighter soils and those prone to wind and water erosion may require postponing tillage until just before seeding to ensure stubble cover for as long as possible.

Alternate host sites, such as field margins, fence lines, pastures, shelterbelts and riparian areas, will usually contain weeds and natural vegetation that may serve as reservoirs for disease, vectors of disease and insect pests. Left uncontrolled, these insect and disease pests can be transmitted to healthy crop plants. Insects may use these plants as alternate habitat until an appropriate crop occurs in a nearby field. However, these areas may also host many beneficial insects and predators, therefore the grower must carefully assess the potential threat from pest insects in these areas before mowing or removing any plants. The ecological importance of areas such as sloughs, wooded bluffs, road allowances, railroad rights-of-way, abandoned farmyards and schoolyards must also be included in long-range planning.

Seed Quality The use of high-quality seed is especially important in preventing disease. The seed supply should be free of smut, ergot bodies or other sclerotia, and free of kernels showing symptoms of Fusarium head blight infection. Seed analysis by a reputable seed testing laboratory will help determine specific diseases in the seed supply. Relatively few diseases are exclusively seed-borne, and it is more common for pathogens to be transmitted from soil, stubble, or wind, as well as with the seed. Planting physically sound seed is also important.  In crops such as flax, rye and pulses, a crack in the seed coat may serve as an entry point for soil-borne micro-organisms that rot the seed once it is planted.

Clean seed sample Photo credit: Canadian Phytopathological Society

Weed Management

Although weeds need to be controlled to reduce their impact on crop yield and quality, a field completely free of weeds is not necessarily the best objective. In many cases, weeds provide food and shelter for beneficial insects. Parasitic wasps, for example, are attracted to certain weeds with small flowers. Field experience has shown that the number of predators attacking insects increases and the number of aphids and leafhoppers decreases on certain crops as the diversity of weeds (that act as host plants) increases. Research has shown that outbreaks of certain crop insect pests are more likely in weed-free fields.

Insects that are generalist feeders, such as beet webworm, thistle caterpillars and grasshoppers, may prefer to feed on weeds rather than some crops, only damaging the crop after the weeds are eaten.

Each field situation should be considered separately, as weed competition must always be taken into account. Sometimes mowing weeds at the edge of the field results in beneficial organisms moving into the crop where they are needed.

Forecasting

Producers should pay attention to the forecasts for various pest infestations for each crop year. Maps of these forecasts are usually available for many of the major destructive insects such as grasshoppers and wheat midge, as well as some diseases. 

Record-Keeping

Keeping diligent field records can provide very useful information. A complete history of each field should include any insect or disease infestations, which management methods worked and which did not, and a list of management techniques to try in the future.

 

Managing the Growth Environment - Giving the Crop a Head Start

Any crop management technique that contributes to a vigorous, competitive crop is a tool of insect and disease management. Producers must also be mindful that many practices that work well in conventional systems may not benefit organic systems. Certain crop species, crop varieties and equipment may work well in one system but not in the other.

Healthy Soil

Maintaining favorable soil conditions is the first line of defense against pests. A biologically active soil with good drainage supports vigorous crop growth, allowing a higher level of crop competition with weeds.

Adequate, balanced soil nutrition is essential for crop quality, yield and moisture-use efficiency. The application of nutrients should be based on a sound soil testing program, accompanied by plant tissue analysis when diagnosing problems. High levels of nitrogen can occur after a high-nitrogen plowdown, such as sweet clover. This results in lush leaf tissue and a dense plant canopy that provides an ideal environment for plant pathogens. However, a lush crop may also help disperse the damage by a given number of insects, so astute observations by the producer are necessary at all times.

In contrast, inadequate soil phosphorous can pre-dispose wheat to certain root diseases. Low levels of nitrogen can reduce the incidence of insect outbreaks. A shortage of micronutrients such as zinc or copper can result in disease-like symptoms on crops, while too much of any one micronutrient may be toxic.

The addition of composted livestock manure improves soil quality, including increasing the population of soil micro-organisms that compete with soil-borne plant pathogens.

Field experience has also shown that plants fertilized by the slow release of nutrients from compost are more resistant to insects and diseases than crops fertilized by highly soluble nutrients. Soil testing becomes important when applying compost regularly. An imbalance of nutrients can easily occur if the soil's nutrient profile is not continuously monitored.

Crop and Variety Selection

Producer awareness of insects and diseases in the proximity of the farm is very important and should influence the crop and the variety of crop to be grown. These choices must also fit in with the crop rotation plan that has been developed. Some insect pests are specific to certain crops, such as wheat stem sawfly, while others, such as grasshoppers, will attack numerous crops. The situation is similar with diseases.
 

Fusarium head blight Photo credit: Canadian Phytopathological Society

Fusarium head blight Photo credit: Canadian Grain Commission

The selection of insect- and disease-resistant cultivars can be a useful tool, but under no circumstances can genetically modified varieties be used in organic systems. Wheat varieties with solid stems are more resistant to wheat stem sawfly than hollow-stemmed varieties. Wheat varieties with resistance to wheat midge have been developed and should soon be available. These insect-resistant varieties were developed through conventional plant breeding programs. Certain species may avoid diseases such as Fusarium head blight, but often agronomic factors, such as time of seeding or choosing winter versus spring wheat, have more of an influence on the incidence of disease.

Plants also vary in their degree of attractiveness to insects, diseases and vectors transmitting disease. Factors such as leaf and stem toughness, pubescence, nutrient content, plant architecture, growth habit and differences in maturity between crops and varieties can influence pest growth, reproduction and host preference. For example, earlier-maturing crop varieties may be less attractive to migrating populations of grasshoppers late in the season compared to later-maturing varieties.

Intercropping

The practice of intercropping (where two crops are grown at the same time) can reduce pest problems by making it more difficult for the pests to find a host crop. This technique also provides habitat for beneficial organisms. Strip-cropping row crops with perennial legumes often leads to better pest control. In particular, alfalfa attracts many beneficial organisms that can destroy insect pests in neighbouring crops. A study in Ontario showed that intercropping corn and soybeans reduced damage by European corn borer. Similarly, it was shown that intercropping tomatoes with cabbage resulted in reduced damage to cabbage by diamondback moth. Studies on corn with soybeans and corn with dry beans have shown significant yield advantages compared to growing the two crops separately.

Seeding Date

Planting should be scheduled so that the most susceptible time of plant growth does not correspond to the peak in pest cycles. Early seeding reduces crop damage caused by grasshoppers, aphids in cereal crops, wheat midge in spring wheat, barley yellow dwarf virus in barley and wheat, powdery mildew in peas and pasmo in flax.

Delayed seeding can be effective in avoiding wireworms and cutworms in cereal crops, Hessian fly in winter wheat, barley thrips, Ascochyta in lentils and wheat streak mosaic virus in winter wheat. However, experience on the Prairies has generally shown that delaying seeding too long can reduce a crop's potential yield.

Seeding Rate

Using a higher seeding rate to affect insect or disease infestations may have different results. More plants in a field may reduce the impact of a given aphid population on individual plants, but they may create a more favourable habitat for insects that prefer a dense canopy, such as true armyworm. A dense leaf canopy can also create a moist soil surface and elevated humidity within the crop, conditions favourable to certain leaf disease pathogens.

Reducing the seeding rate may decrease the severity of take-all in spring wheat, but the reduced canopy may also allow weeds to invade. In other crops, reduced seeding may also produce more insect damage, as in the case of aphids, flea beetles and leafhoppers, which are attracted to the contrast between a green host and a dark soil background.

Depth and Timing of Seeding

Optimum seeding depth is also important. Deep seeding in cold soils may result in seedling blights and damping-off, especially in pulses and small-seeded crops. Seeding depth should generally be no deeper than required for quick germination and even emergence. Variables include seed size, soil type and moisture conditions. If the soil is loose before seeding, a packing operation will firm up the soil and bring moisture closer to the surface.

For most crops, seeding should ideally be done when the soil is warm enough for rapid germination. Seeds that remain ungerminated in cool soil are more susceptible to damage by insects such as wireworms.

Trap Strips

Seeding trap strips around the edge of a cropped field or along a fence row helps lure insect pests to a specific area where they can be managed more easily. For example, planting bromegrass near a wheat field attracts wheat stem sawflies and their native parasites away from the wheat crop. Similarly, a trap strip of potatoes planted much earlier than the main crop would attract Colorado potato beetles to the area. The strip could be worked under along with the adult beetles, eggs and larvae before the second generation of beetles spreads to the main crop.

Generally, the insect pests in the trap strips are controlled by mowing or cultivating the strip, or by applying an acceptable organic product, such as Bacillus thuringiensis. Trap strips can also act as a barrier to protect the crop field. Producers have found that planting yellow sweet clover or Sirius field peas repels grasshoppers and prevents them from damaging crops. A thorough knowledge of the crop and insect pests of the area is necessary to prevent this technique from backfiring.

Tillage

Tillage can be properly timed before seeding, after harvesting and during summerfallow to reduce populations of insect pests such as cutworms and grasshoppers that spend part of their life cycles in the soil or stubble. Tillage can help starve insects in the spring or during fallow, prevent adults from laying eggs in the soil and expose overwintering insects to predators and inclement weather.

Roguing

Roguing refers to the labour-intensive practice of walking the fields to remove diseased or insect-infested plants. Roguing may not be practical for large fields, but could be suitable for seed plots or crops having highly infectious and destructive diseases (for example, bacterial blackleg in potatoes and certain viruses in other crops).

Direct Treatment

At times, the organic producer will find that, despite all the best efforts, an insect or disease pest will grow to levels that cause substantial crop damage. At this point, direct treatment may be necessary.

Monitoring

Insect monitoring traps are useful in determining which insect pests are present in a field and whether they are at economically important levels. It is imperative that the producer has a positive identification of the insect or disease causing damage before choosing a method of treatment.

Certain types of insect hormones called pheromones may be used as attractants to monitor population levels of insects such as bertha armyworm, diamondback moth, cabbage looper and European corn borer, or to simply attract insects into a trap.

Biological Control

In a healthy, balanced ecosystem, biological control by natural predators is constantly occurring. The more diverse a cropping system becomes, the greater the spectrum of insect species and micro-organisms within it. This leads to the development of more natural predators within the ecosystem.
 

Beneficial insect: lacewing Photo credit: Roy Ellis

Beneficial insect: parasitic wasp Photo credit: Roy Ellis

Ladybugs, ambush bugs, hoverfly larvae, lacewings, spiders, birds, frogs, toads and a host of other insects are predators of aphids, bertha armyworm larvae, sunflower beetles, beet webworms, and both grasshopper eggs and adults. The destructive wheat midge may also be partially controlled by a parasitic wasp, but crop damage may still occur.

Various types of fungi are insect parasites and can either kill their insect hosts or reduce their ability to reproduce. A naturally occurring fungal disease of grasshoppers is favoured by moist weather and has been effective in destroying large populations of grasshoppers in many areas of the Canadian Prairies. Other fungi attack certain aphid species and canola root flies.

Very few biological controls are available to reduce the effects of plant diseases, as most commercial products do not perform well if the disease is already established in the crop. Mycoparasitism is a form of bio-control where one fungus parasitizes another. Although this process occurs with many fungi under laboratory conditions, it hasn't been successful under field conditions.

Natural Insecticides

Organic certification standards prohibit the use of synthetic pesticides. Permitted disease-management products include copper (fixed copper and copper sulphate), lime-sulphur mixes, elemental sulphur, vinegar, soap and silica. Bordeaux mixture is considered a restricted substance, and farmers should contact their certifying body before using it. Although these products are allowed, it may not be cost-effective or feasible to apply them to field crops. Scientific evidence on product efficacy should be researched before using them.

The high risk of phytotoxicity should also be considered when using these products on certain plants; often the margin of error between benefit and damage to the plant is very small. In addition, there are environmental and ecological concerns surrounding some of these products. Additional soil tests may be required to monitor copper and sulphur levels in the soil. As well, organic certification may be denied to farms that overuse or depend on such products.

Insecticides permitted in organic agriculture include some microbial insecticides containing the bacteria Bacillus thuringiensis. Three main strains of these bacteria are used in insect control. One strain, marketed as Dipel or Thuricide, kills only the larvae of moths or butterflies. Another strain, marketed as Novodor, is for beetle larvae only and can be used to control Colorado potato beetles. The third strain is specifically for mosquito and fly larvae. Botanical insecticides, such as rotenone, are also permitted in organic agriculture, but they are often too expensive to use on large acreages. Refer to the label for specific instructions before using any pesticide. Labels for pesticides, including those permitted in organic agriculture, can be found on the Pest Management Regulatory Agency website at http://eddenet.pmra-arla.gc.ca/4.0/4.0.asp

Colorado potato beetle larva Photo credit: Roy Ellis

Colorado potato beetle adult   Photo credit: Roy Ellis

Other Control Methods

For pest control, the following can be used: beneficial organisms, dormant oil, diatomaceous earth, plant-derived pesticides, soap, natural and synthetic insect pheromones which disrupt the insect's development, and commercial insect vacuums. Restrictions apply to some of these products, and the products are not always 100 per cent reliable. Farmers should contact their certifying body before using any product on their crop.

By creating diverse eco-systems on the farm, producers will observe the effect on pests of domestic and wild birds and numerous vertebrate predators such as moles, shrews, bats, mice, skunks, toads and frogs.

Grain Storage

When stored grain is dry and its temperature is low, problems seldom arise. But if the grain is warm and moist, insects and fungi can multiply rapidly. A grain temperature of 5°C to 10°C is adequate for long-term storage. Bin aeration helps dry and cool the grain. If bins are not equipped with aeration systems, grain can be moved to cool it. Cold temperatures can be used to control insects that exist in stored grain. Information on using cold temperatures to control insects in stored grain can be found at www.gov.mb.ca/agriculture/crops/cropproduction/faa06s00.html

Before storing new grain, the bin should be thoroughly cleaned with a grain vacuum. The empty bin can be treated with diatomaceous earth to control stored-grain insects. Many organic producers have found it to be effective when applied as a light coating on the floor of the bin, and around the walls and the bin door. It can also be added to the grain as it is being brought into storage.

Products containing diatomaceous earth are registered for use in grain handling and storage areas. It is important that producers read the label carefully, as the rate used will vary with the type of application. The test weight of a bushel of grain can also be changed by the addition of the product. It works as a contact insecticide and has been found to be most effective in wheat and barley, although many maltsters prohibit its use in malting barley.

Conclusion

Effective, ecologically sound insect and disease management programs require a high level of grower knowledge and management.

Pest organisms and their parasites, host crops, associated vegetation and soil conditions are all elements of an interdependent system. The integration of insect and disease control methods and crop production practices must be done carefully and suit the producer, location and type of cropping system. Pest management systems must be devised to utilize control methods that work harmoniously within the cropping system while maintaining the pest populations below economically damaging levels.

References

• Organic Field Crop Handbook. 2nd ed. Ottawa: Canada Organic Growers, 2001.
• Farm Facts - Organic Crop Production: Disease Management. Saskatchewan Agriculture, Food and Rural Revitalization, March 1996.
• Farm Facts - Organic Crop Production: Insect Management. Saskatchewan Agriculture, Food and Rural Revitalization, March 1996.
• Organic Agriculture Web Links. Manitoba Agriculture, Food and Rural Initiatives. http://www.gov.mb.ca/agriculture/organic/org02s08.html
• Field Crop Production Guide. Revised ed. Manitoba Agriculture, Food and Rural Initiatives, 2001.
• Earthcare: Ecological Agriculture in Saskatchewan. Muenster, SK: Earthcare Group , 1980.
• Practical Crop Protection. Alberta Agriculture, Food and Rural Development, 1994.

For further information, contact your GO representative.